Question

我有3个表：broadcasts，broadcasts_likes和broadcasts_viewers：

我想要返回结果包括total_likes，total_viewers但是当我运行这个sql时，期望是错误的，你能帮我检查一下吗？我只是想正常使用sql，不要使用子查询。这是我的sql：

<manifest ...>

  <!-- Include required permissions for Google Mobile Ads to run-->
  <uses-permission android:name="android.permission.INTERNET"/>
  <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE"/>

  <application>

     //...

     <!--Include the AdActivity configChanges and theme. -->
     <activity android:name="com.google.android.gms.ads.AdActivity"
      android:configChanges="keyboard|keyboardHidden|orientation|screenLayout|uiMode|screenSize|smallestScreenSize"
      android:theme="@android:style/Theme.Translucent" />
  </application>
</manifest>

Answer 1

最简单的解决方案是切换到mov ecx, msg：

count(distinct)

如果广播有很多观点和很多喜欢的话，那么这个节目不会特别好。在这种情况下，您可以在加入或使用相关子查询之前聚合表：

SELECT `b`.*, 
       COUNT(DISTINCT `bl`.profile_id`) AS `total_likes`,      
       COUNT(DISTINCT `bv`.view_profile_id) AS `total_viewers` 
FROM `broadcasts` `b` LEFT JOIN
     `broadcasts_likes` `bl` 
     ON `b`.`broadcast_id` = `bl`.`broadcast_id` LEFT JOIN
     `broadcasts_viewers` `bv` 
     ON `b`.`broadcast_id` = `bv`.`broadcast_id` 
GROUP BY `b`.`broadcast_id`;

实际上，对于SELECT `b`.*, (SELECT COUNT(*) FROM broadcasts_likes bl WHERE `b`.`broadcast_id` = `bl`.`broadcast_id` ) as `total_likes`, (SELECT COUNT(*) FROM broadcasts_viewers bv WHERE `b`.`broadcast_id` = `bv`.`broadcast_id` ) as `total_viewers` FROM `broadcasts` `b`;和broadcasts_likes(broadcast_id)上的索引，这应该具有非常好的性能。

计数和加入3表时错误的期望

1 个答案: